Yes, the necro can damage with minions, so he doesn't have to himself. But his damage is only greater if he gets into the action. And if he doesn't, then it makes it easier for foes to kill off his minions. So the necro really has to stay in the battle to be most effective.
As you say, thieves depend on hit-and-run attacks. A thief without initiative would do a lot more damage, a lot more than warriors or guardians I would guess. That would get nerfed fast because doing too much damage is frowned upon. So they would really crank up the skill recharge rate, so you would no longer be able to unload twice in a row. So the combat would now feel very much like warrior or guardian or ranger. Then, he would die all the time because of his lower life and lower armor, so they would buff that.
And then you have another way to play warrior. Boring? Yes. You might disagree. But a lot of people would not enjoy the new thief. This is what happened with the assassin, and look, it was a lot of problems.
Right now the profession is interesting because it is different from other professions. The initiative really gives you a reason to hit & run, which is what the thief should be doing.